Bulk Acoustic Waves (BAWs) are generated in piezoelectric media by applying a time-varying electric source to a suitable electrode system. The earliest uses of BAWs were in BAW plates, but they were found to be inadequate for higher frequencies because of difficulties in grinding the plates to the requisite thinness. Then, Surface Acoustic Waves (SAWs) were tried for the higher frequency ranges, but SAWs could not be generated efficiently, until the Interdigital Transducer (IDT) was introduced. The IDT had the estimable quality of generating SAWs by its geometry. The IDT finger spacing determined the main operating frequency of the SAW device, because it determined the wavelength, and hence frequency, at which resonance occurred. Because the IDT photolithography means, and because very small finger spacings could be obtained thereby, the IDT afforded the ability to reach high frequencies simply by depositing a metallic finger pattern on the surface of the thick, robust substrate. The surface wave “stuck” to the surface, and would not penetrate much into the depth of the substrate. There was no need to grind the substrate into an impossibly thin layer.
Yet, even SAWs suffer from a number of limitations such as beam divergence and triple-transit echoes and the possibility of making BAWs at frequencies to match, or even exceed, those of the SAWs has been investigated. Making BAWs to match the SAW frequencies would require techniques other than mechanical grinding, because the requisite thin dimensions were far too small to grind. Another approach was to fabricate a BAW with thin film. Up until now, IDTs have not been employed to generate BAWs in the thin film because the IDT is generally considered a modality for generating SAWs, not BAWs.
When BAWs are used as an acoustic resonator or filter, one or more of the frequency-determining dimensions of the piezoelectric body yield the desired resonance characteristics. Considering the case of a thin plate made of a single piezoelectric material with electrodes placed on the major surfaces, if the requisite piezoelectric coefficient exists when an electric voltage energizes the plate, it exhibits resonances when the exciting frequency is such that its thickness dimension is an odd integral multiple of one-half of an acoustic wavelength. In this situation, the BAW forms a standing wave pattern in the thickness direction and provides two counter-propagating waves that travel in the thickness direction. Communications technology continues to seek devices operating at increasingly higher frequencies. Wavelength is inversely proportional to frequency; therefore one finds increased use of the thickness of piezoelectric materials as the frequency-determining dimension for BAWs with thin film and membrane structures reaching the necessary higher frequencies. These mechanical motions are referred to as thickness modes of vibration. One excitation method is the “thickness excitation” (TE) or “thickness-field excitation” (TFE). When electrodes are placed at the plate edges and produce a field in a direction parallel to the plate surface, and hence perpendicular to the acoustic plane wave direction, this is known as “lateral excitation” (LE) or “lateral-field excitation” (LFE).
Besides exciting resonators with the SAW generated with an IDT, shallow bulk acoustic waves (SBAWs) or surface skimming bulk waves (SSBWs) have also been used. One characteristic of the SBAW and the SSBW is that surface perturbations, such as corrugations and ridges, placed parallel to the electrode fingers, but outside the IDT region, keep the quasi-BAW waves bound in the region of the surface so they do not escape into the substrate interior. However, devices based on the SBAW and SSBW wave types are not commercially viable. Until now, BAWs generated from IDT would be considered weak, spurious and detrimental because the typical low IC voltages do not provide adequate electric field strength for piezoelectric excitation of conventional electrode structures and result in unacceptable performance. Additionally, a non-uniform electric field further degrades performance. Up until now, the IDT has never been used purposely to generate plane-wave BAWs that propagate away from, and normal to, the substrate surface.
To overcome the disadvantages, shortcomings and limitations of the prior art resonating structures, there has been a long-felt need to provide a planar electrode structure that is both IC-compliant and able to be energized by a low-voltage source. It is also critical to achieve an adequate electric field strength that is uniformly distributed over the whole BAW accessible active region. The necessary increased voltage is generally inconsistent with the voltages resident on typical IC chips and in other electronic components. Electric field strength, which produces piezoelectric driving, equals, for a uniform field, applied voltage divided by the electrode separation. For a given applied voltage, requisite electric field strength determines the gap size, or separation. Prior art LFE electrode configurations employ individual electrode pairs placed on, or recessed in, a resonant substrate, causing a relatively large separation. The present inventors have developed a quite different technique for resolving the long-standing problems, difficulties and disadvantages of inadequate excitation strength and non-uniform power distribution.
The present invention provides an advantageous and innovative IDT structure to generate BAWs by modifying the IDT with dielectric structures to make the IDT capable of generating BAWs efficiently. Instead of depositing two electrodes exposing a large fraction of the active area of the resonant structure to the exciting electric field, this invention provides for depositing two exciting IDT electrode structures with finger spacings for LFE sufficiently close together on the piezoelectric substrate to cause a voltage compatible with IC devices for a usefully high electric field strength, which results in a substantial region of excitation covered by the IDT finger structure so as to produce efficient transduction. Depositing the exciting electrodes on the piezoelectric substrate in this manner results in an electrode system that is properly matched to the piezoelectric structure and overcomes the difficulties, disadvantages and shortcomings of the prior art piezoelectric resonators and frequency discriminators.